Compositions for colouring or dyeing hair can be categorised into temporary, semi-temporary, semi-permanent, permanent and, more recently, the so-called demi-permanent products. Although these names try to describe the general longevity of the colour in terms of wash-out, there is considerable blurring of the categories depending upon the brand of hair colour purchased, the dyes used and the formulation of the carrier base. However, as a guide:                Temporary lasts only one wash        Semi-temporary lasts approximately 5 washes        Semi-permanent lasts approximately 10 washes        Demi-permanent last approximately 20 washes        Permanent is not washed out and lasts until the colour fades or grows out        
Other products on the market are sold as post-colour formulations and these claim to extend the life of the hair colour. These can be split into ‘colour maintenance’ products, normally shampoos, which are formulated to be mild and help reduce colour washout, and ‘re-tint’ products, which possess a low concentration of colour to reinforce the existing colour of the hair.
The chemistry of hair dyes differs greatly. Temporary and semi-temporary colours tend to be larger molecules that are unable to penetrate the hair shaft and so reside on the hair surface. This makes them easy to remove with washing. Semi-permanent, demi-permanent and permanent colours are smaller molecules, which are able to penetrate the hair shaft, making them more resistant to hair washing.
The following list is a guide to the types of dye often found in the particular category of hair product. It should be noted that many commercially available products contain combinations of dye types:                Temporary hair colours include food or vegetable dyes or their insoluble lakes (pigments). Here, the colouring formulation normally contains fixatives such as resins to hold the colour onto the hair. These are easily removed with shampoo.        Semi-temporary dyes are cationic in charge. The cationic charge allows them to bind to the hair surface and makes them more resistant to washing.        Semi-permanent dyes are normally small, nitro-dyes, which are able to penetrate the hair shaft. This makes them less readily removed with washing.        Permanent dyes (and couplers) are themselves colourless precursors called oxidation dyes. These precursors, in the presence of peroxide, undergo a chemical reaction to produce coloured compounds of varying degrees of polymerisation. Penetration of the colourless precursors into the hair fibre can be ‘encouraged’ by the use of an alkali, such as ammonia, which causes the hair shaft to swell and become more porous. Once within the hair fibre, the small colourless precursors polymerize to form the larger coloured compounds. These are then trapped within the hair fibre by their physical size.        Demi-permanent colours use similar, colourless, precursors to the permanent colours. However, the use of different coupling agents restricts polymerization to the formation of smaller coloured compounds. These smaller compounds are more easily washed out of the hair. Also, the reduction or elimination of ammonia from the formulation helps to limit the penetration of the precursors.        
Many products contain a combination of different dye types. For example, some semi-temporary dyes are mixed with the semi-permanent dyes to produce more natural looking shades. The demi-permanent and permanent products can provide a wide variety of colours even on dark hair because the natural hair colour is bleached during the process. The use of the temporary, semi-temporary and semi-permanent colours are dependent upon the individual's natural hair colour; a dark haired person cannot become blond.
The reason for the recent popularity in demi-permanent colouration comes from the desire to produce a more natural look and reduce damage to the hair. Demi-permanent colouring is also used to boost an old, fading, permanent colour.
Semi-temporary or semi-permanent products are used to tint the hair. A growing market for these colours is also in masking gray hair. This is because these types of colours effect the lighter (white) hairs more than the darker hairs, helping to provide a more natural look. However, as described above, such products do not last very long.